add set-operators.md (#4070)

* add set-operators.md

* refine format

* Apply suggestions from code review

Co-authored-by: TomShawn <[email protected]>

* Update functions-and-operators/set-operators.md

Co-authored-by: TomShawn <[email protected]>

TOC.md

@@ -387,6 +387,7 @@
       + [Window Functions](/functions-and-operators/window-functions.md)
       + [Miscellaneous Functions](/functions-and-operators/miscellaneous-functions.md)
       + [Precision Math](/functions-and-operators/precision-math.md)
+      + [Set Operations](/functions-and-operators/set-operators.md)
       + [List of Expressions for Pushdown](/functions-and-operators/expressions-pushed-down.md)
     + [Constraints](/constraints.md)
     + [Generated Columns](/generated-columns.md)

functions-and-operators/set-operators.md

@@ -0,0 +1,131 @@
+---
+title: Set Operations
+summary: Learn the supported set operations in TiDB.
+---
+
+# Set Operations
+
+TiDB supports three set operations using the UNION, EXCEPT, and INTERSECT operators. The smallest unit of a set is a [`SELECT` statement](/sql-statements/sql-statement-select.md).
+
+## UNION operator
+
+In mathematics, the union of two sets A and B consists of all elements that are in A or in B. For example:
+
+```sql
+select 1 union select 2;
++---+
+| 1 |
++---+
+| 2 |
+| 1 |
++---+
+2 rows in set (0.00 sec)
+```
+
+TiDB supports both `UNION DISTINCT` and `UNION ALL` operators. `UNION DISTINCT` removes duplicate records from the result set, while `UNION ALL` keeps all records including duplicates. `UNION DISTINCT` is used by default in TiDB.
+
+{{< copyable "sql" >}}
+
+```sql
+create table t1 (a int);
+create table t2 (a int);
+insert into t1 values (1),(2);
+insert into t2 values (1),(3);
+```
+
+Examples for `UNION DISTINCT` and `UNION ALL` queries are respectively as follows:
+
+```sql
+select * from t1 union distinct select * from t2;
++---+
+| a |
++---+
+| 1 |
+| 2 |
+| 3 |
++---+
+3 rows in set (0.00 sec)
+select * from t1 union all select * from t2;
++---+
+| a |
++---+
+| 1 |
+| 2 |
+| 1 |
+| 3 |
++---+
+4 rows in set (0.00 sec)
+```
+
+## EXCEPT operator
+
+If A and B are two sets, EXCEPT returns the difference set of A and B which consists of elements that are in A but not in B.
+
+```sql
+select * from t1 except select * from t2;
++---+
+| a |
++---+
+| 2 |
++---+
+1 rows in set (0.00 sec)
+```
+
+`EXCEPT ALL` operator is not yet supported.
+
+## INTERSECT operator
+
+In mathematics, the intersection of two sets A and B consists of all elements that are both in A and B, and no other elements.
+
+```sql
+select * from t1 intersect select * from t2;
++---+
+| a |
++---+
+| 1 |
++---+
+1 rows in set (0.00 sec)
+```
+
+`INTERSECT ALL` operator is not yet supported. INTERSECT operator has higher precedence over EXCEPT and UNION operators.
+
+```sql
+select * from t1 union all select * from t1 intersect select * from t2;
++---+
+| a |
++---+
+| 1 |
+| 1 |
+| 2 |
++---+
+3 rows in set (0.00 sec)
+```
+
+## Parentheses
+
+TiDB supports using parentheses to specify the precedence of set operations. Expressions in parentheses are processed first.
+
+```sql
+(select * from t1 union all select * from t1) intersect select * from t2;
++---+
+| a |
++---+
+| 1 |
++---+
+1 rows in set (0.00 sec)
+```
+
+## Use `Order By` and `Limit`
+
+TiDB supports using [`ORDER BY`](/media/sqlgram/OrderByOptional.png) or [`LIMIT`](/media/sqlgram/LimitClause.png) clause in set operations. These two clauses must be at the end of the entire statement.
+
+```sql
+(select * from t1 union all select * from t1 intersect select * from t2) order by a limit 2;
++---+
+| a |
++---+
+| 1 |
+| 1 |
++---+
+2 rows in set (0.00 sec)
+```